The main objective of this project has been the systematic probing of nucleoside/nucleotide binding enzymes with sets of conformationally rigid substrates to learn about their preferred binding mode. This has been accomplished by constructing carbocyclic nucleoside analogues on a bicyclo[3.1.0]hexane template which essentially removes the inherent flexibility of the sugar moiety by locking the conformation of the sugar in one of the two preferred natural conformations (North or South). A secondary objective of this project has been the construction of modified nucleic acids that incorporate a number of locked nucleotide units to either reinforce or disrupt the typical B- or A-DNA conformations associated with South and North conformations, respectively. The major findings are: 1) The corresponding 5'-triphosphates of the locked antipodes of adenosine (N-MCdA and S-MCdA) showed that that the most effective HIV RT inhibitor (N-MCdA-5'-triphosphate) inhibited DNA elongation beyond the polymerase site and showed great effectiveness against all HIV-resistant strains that function by the excision mechanism. This approach offers a novel and promising way to overcome HIV resistance. 2) A series of short oligodeoxynucleotides (ODNs) corrresponding to the self-complementary EcoR1 recognition sequence [ds(5'-CGCGAATTCGCG-3')] where the middle A's and the middle T's have been replaced by locked adenosines and locked thymidines have been synthesized and the structures solved by NMR, CD and calorimetry. The results will be published shortly. 3) A novel synthesis developed for North conformers involving a lipase-catalyzed resolution step of the bicyclo[3.1.0]hexane system was very successful and reproducible in a preparative scale by a contractor. Supplies of all building blocks for additional DNA syntheses will be forthcoming.